Amplifying device



Mair 31, 1938. K. POSTHUMUS I 2,119,351

AMPLIFYING DEVICE Filed March 16, 1954 I I .J. I I

I a I l l -I INVENTOR I KlAAs POsTHuMus ATTORNEY Patented May 31, 1938 PATENT OFFICE AMPLIFYING DEVICE Klaas Posthumus, Eindhoven, Netherlands, assignor to Radio Corporation of America, a corporation of Delaware Application March 16, 1934, Serial No. 715,792 In the Netherlands April 5, 1933 '7 Claims.

This invention relates to amplifying circuit arrangements comprising thermionic tubes for receiving or transmitting electric oscillations, in which impedances are inserted in that part of the cathode line common tothe circuits connected to a tube like the controlling grid-screen gridanode circuit.

It is known that a certain conductivity occurs between the controlling grid and the cathode of a thermionic tube, even when the negative grid bias is so chosen that no electron current can flow between said electrodes, which conductivity, if a tuned circuit is inserted in the controlling grid circuit, causes a certain damping. As is known to those skilled in the art this conductivity at high radio frequencies may be due to electron transit time phenomena and at low frequencies may be caused by the effect of the grid-anode capacity when an external load impedance is included in the anode circuit, as is usual with such amplifiers. Simultaneously therewith a gridcathode capacity also depending on the negative controlling grid voltage is present between controlling grid and cathode, which capacity may substantially be ascribed to variable space charge phenomena.

Means have been provided for neutralizing or compensating these phenomena entirely or partly which was achieved by strongly reducing the self-induction of the cathode lead of the thermionic tube which is common to the circuits connected to the tube, and furthermore by inserting impedances, for instance ohmic resistances and condensers, in the cathode lead.

The present invention has for its purpose to improve the selectivity of high frequency amplifiers, in which a resonance circuit is interposed in the controlling grid circuit of a thermionic tube. According to the invention this is achieved by giving the condenser, inserted in the part of the cathode lead common to the grid circuit or circuits and to the anodecircuit, such a value that a resonance circuit inserted in the controlling grid circuit is undamped.

For this purpose the use of impedances in the common cathode line with triodes has been suggested, and the use thereof was also proposed for valves having a plurality of grids, for instance screening grid tubes, in which case the expression for the apparent admittance existing between the controlling grid and the cathode, is

trol grid. A similar formula applies in the case of a screen grid tube,

if in this expression the anode-controlling-grid capacity Ca of the triode were replaced by the screening grid-, controlling grid capacity of the tetrode, the impedance Za in the anode circuit of the triode by theimpedance in the screening grid circuit and if for the slope S of the triode the slope of the screening grid characteristic of the tetrode were substituted; Thus it was supposed that the anode was directly connected to the cathode so that the anode-alternating current did not flow through the impedances in the common part of the cathode lead, or if this was nevertheless the case that the reaction caused by the anode currents on the controlling grid circuit could be neglected.

According to this application there is connected in the cathode lead common to the control grid-and anode circuit of an amplifier tube an impedance consisting of a condenser C and a resistance R in series.

In order to explain the effect of this impedance on the impedance between the terminals l and 2 in Fig. 1 of the application reference is made to the formula:

wherein Z indicates the impedance measured between the terminals l and 2 in absence of the tuned circuit Zg constituted by Lg and Cg;

S is the slope of the tube characteristic; Zr=the impedance in the common cathode lead; c the control grid-cathode capacity; Za=the impedance in the anode circuit; Ca=the anode-control grid capacity; If now in the above formula for Z: is substituted ZI=R we obtain 1 C 2 Sa-'- JwSRC,,+JwSZ,,C

The first term is and represents a negative conductance so that if there is connected a tuned circuit Zg between the terminals l and 2 in Fig. 1 this tuned circuit is undamped by the negative conductance so that the resonance curve of this circuit is sharpened and a better selectivity is obtained. The second term; 7iw,SR-Cg represents a negative capacity between theterm-inals and 2. As is well known the control grid cathode capacity of an amplifier tube is dependent on the space charge between said electrodes which in turn is dependent on the slope of the characteristic. So when the grid bias of such tube is varied, for example, for volume control purposes the control grid capacity varies with the bias (slope). This has a detuning effect on the tuned circuit Zg between the terminals I and 2. Thisis compensated by the negative capacity caused by the resistance R as it will be "seen that this negative capacity is also dependent on the slope S. 1 With modern screening-grid tubes, however, having an internal resistance of the order of magnitude'of 10 ohms the impedance in the anode circuit is practically always much smaller than the internal resistance of the tube, so that the anodecurrent is almost completely determined by the slope of the anode-current characteristic and the alternating potential set up at the controlling grid; It will be appreciated that in such cases the reacting effect of the anode current .on the controlling-grid circuit through the impedances inserted in that part of the cathode line common'to controlling grid-, screening grid-', and anode circuits, can no longer be neglected. This means, however, that the impedances in the common part of the cathode lead must be given other values, whilst the working of these impedances remains the same, so that in this case .the apparent conductivity of the grid circuit as well'as the variable controlling-grid cathode capacity can be neutralized. r

In order to neutralize the apparent conductivity the capacity in-the common .part of the cathode line must be so large that it produces an equally large negative conductivity between the connecting terminals connected to the cathode and the controlling grid. V

Qnemay go still further, however, by giving the capacity such a value that the apparent negative conductivity, as regards itsabsolute value, is largerthan the positive conductivity so that a negative conductivity results between the grid and the cathode terminal. In this case it is possible to undamp a resonance circuit connected between these terminals, owing to which both the selectivity and the amplification of the high frequency amplifier can be improved. For a better understanding of the invention reference is made to the accompanying drawing, in which,

Figs. 1 and 2 are schematic circuit diagrams of tube amplifiers embodying the invention and,

Fig. 3 shows curves showing the variation of the conductivity between the terminals I--2 of Fig. 2 with variation of condenser C.

The invention will be more clearly understood by reference to the accompanying drawing. Fig. 1 represents the circuit arrangement of a. high frequency amplifier according to the invention. The arrangement comprises an indirectly heated screening grid tube provided with a cathode K,

a controlling grid G, a screening grid S and an anode A (the heater not being represented). Said electrodes are connected to terminals l, 2, 3 and 4 located, for instance, in the base of the tube. An impedance Z; consisting, for instance, of the parallel connection of an inductance Lg and a condenser Cg is inserted between the terminals l and an impedance Za also consisting of a parallel connection of an inductance La and a condenser Ca lying between the terminals 2 and 4. By means of the condensers Cg and Ca the grid and anode circuits are tuned to the desired frequency. The terminal 3 connected to the screening grid; is short-circuited to terminal 2, as high ire uency currents only are considered here. The line connecting the cathode K to the terminal 2 comprises a series-connection of a condenser C and an ohmic resistance R with which an inductancecoil L serving for the passage of the'anodeand screening-grid direct current is connectfed'in parallel. The resistance R between the terminals K and 2 causes a negative capacity which depends on the'negative controlling grid voltage 'in same manner as the controlling grid-icathodecapacity depends on space charge phenomena, and whichsubstantially compensates the latter. -If':the' condenser C were used alone in the common portion of the grid and anode circuitsit would produce much more regenerationin the circuit Zg at high frequencies than at low frequencies. The addition of the resistor R in this'circuit causes this effect to be less pronounced at high frequencies and thereby makes the regeneration over the frequency range to be amplified more uniform. Another function of the resistor R is to produce a voltage 180 out of phase with the voltage applied between terminalsl' and "2 so that currents flowing between grid 'Gancl cathode K are decreased, thus causing the g'rid circuit to act as though the control gridto cathode capacity were decreased. This effect can 'be'inade to substantially compensate the increase in grid to cathode capacity due to space charge phenomena. Also as the control grid negative bias is varied, the space charge capacity and :the effect of voltages across R clue to the plate current therethrough on the apparent input capacity between terminals I and 2 will vary in opposite sense,- thus maintaining an apparent constant input. capacity across the tuned circuit Zg with change in the control grid bias. v

The condenser C inserted in the lead connecting the cathode K .to the terminal 2 causes a negative conductivity between the terminals l and 2 which conductivity counteracts the positive conductivity brought about by the reaction of the screening grid through the screening grid-controlling grid capacity. If the condenser C is given such a valnethat therresulting apparent conductivity present between the terminals l and 2 is negative, then the resonance circuit Zg will be undamped. This entails the advantage that a coil 'Lg of poor quality suffices, without the amplification factor of the high frequency amplifier and its selectivity being smaller than if coilswere used which are free of losses. Coils of inferior quality can be made materially smaller than coils free of losses. Moreover, the screening becomes simpler when using a plurality of circuits and in this manner a material saving in set building is achieved. Preferably, the coils are constructed in such a manner, for instance, by furnishing them with an iron core, that in combination with a variable condenser they constitute a tuned circuit whose impedance only slightly varies within a large tuning range. By using such circuits in the arrangements according to the invention it results that the undamping of this circuit over a large range of wave lengths is always maintained substantially constant by inserting the condenser C in the common cathode line.

In the arrangement represented in Fig. 1 the whole of the tuned circuits Zg and Za is connected between the terminals l, 2 and 2, 4 respectively. Usually the connection of these circuits to the amplifying tube occurs in another manner viz. by providing the coils Lg and La with tappings which are connected to the grid and the anode respectively. With arrangements in which, according to the invention, impedances are inserted in the common cathode line, the connection of the controlling grid to a tapping of the'coil Lg would cause difiiculties, for in this case, in order to undamp the whole of the circuit Zg in a conclusive manner the apparent negative conductivity between the terminals I and 2 should be greater, which entails difficulties not only when adjusting the condenser C but which, moreover, gives rise to the condition wherein the coil part c; between the terminals l and 2 together with the controlling grid-cathode capacity between said terminals begins to self-oscillate at a high frequency. Therefore, as to the grid circuit it is advisable always to connect the whole of the resonance circuit Zg between the terminals l and 2 and to connect only the anode to a point of the coil La which lies more towards the end of this coil remote from the cathode.

The usual control of the sound strength by variation of grid voltage is not possible without further means with the arrangements according to the invention, which will be appreciated when considering that the apparent negative conductivity brought about by the condenser between the terminals 1 and 2 depends on this condenser and moreover on the slopes of the anodeand the screening grid capacity, which slopes vary with the variation of the negative controlling grid bias. In this case this would be incidental to a modification of the apparent negative conductivity, so that the circuit Zg between the terminals I and 2 would be undamped more or less which is not desired in View of the selectivity.

Fig. 2 shows a circuit arrangement of a high frequency amplifier forming part of a radio receiving set adapted for receiving two ranges of wave lengths. The arrangement represented in Fig. 2 is similar to that shown in Fig. 1 with the difference that in Fig. 2 three switches S1, S2 and S3 are provided.

The switches S1 and S2 serve for passing over from one range of wave lengths to the other. For the reception of short waves these switches are closed thus short-circuiting parts of the coils La and Lg. The switch S3 is closed when receiving long waves thus causing a short-circuit of the impedances inserted in the connecting line between the cathode K and the terminal 2. In

fact, it appears that the negative conductivity between the terminals l and 2 varies for short and long waves with the size of the condenser C according to thecurves a and 17 shown in Fig. 3 inwhich curve a is for the shortwave position and curve h for the long wave position. From these curves it appears that with long Waves the negative conductivity becomes very small for a definite adjustment of Hg by the condenser C indicated by the dotted line, which adjustment has proved to be adequate for the short waves. When amplifying long waves it is better in this case to short-circuit the impedances in the cathode line which may be effected by a switch S3 connected as shown in Fig. 2.

With the arrangements shown in Figures 1 and 2 it was taken for granted that the terminal 3 connected to the screening grid was short-circuited with the terminal 2 as far as high frequency currents were considered. It would also be possible to connect the screening grid directly in for high frequencies to the cathode. In this case, however, there is a great risk that the arrangement may be caused to self-oscillate by the reacting influence exerted by the anode circuit on the controlling grid circuit through the intermediary of the impedances R and C in the common cathode line and the screening grid to anode capacity. In order to avoid this it is consequently necessary to connect the screening grid terminal 3 and the terminal 2 in the manner shown in Figures 1 and 2 as by a short lead 6.

In case thermionic tubes are used comprising more than two grids or in case the tube is equipped with a metal envelope (for instance by Schoops method) which must be connected for high frequencies to the cathode, said connections must not be provided at the cathode K but at the terminal 2 (Figures 1 and 2).

The condenser C and the resistance R in Figures 1 and 2 may be placed either in the amplifying tube itself or in the parts of the arrangement at the outside thereof and may be furnished with adjustable correction means. This is of importance in order to be able always to adjust the desired value of the negative conductivity and of the negative capacity in the grid circuit when interchanging the amplifying tubes. However, the correction of the negative conductivity can also be achieved by modification of the bias of the controlling grid.

I claim:

1. In an amplifying device, the combination of a vacuum tube having a cathode, a control grid and an anode, an anode circuit connecting said cathode and anode, a grid circuit resonant to a desired frequency connected to said control grid and cathode and the series connection of a condenser and resistor common to said grid and anode circuits and so arranged as to pass the alternating component of the current of said anode circuit through both said resistor and condenser.

2. The combination defined in the preceding claim in which said condenser is adjustable to such avalue as to compensate for the losses of said resonant grid circuit.

3. In an amplifier, the combination of a vacuum tube having a cathode, a control grid and an anode, a grid circuit resonant to a desired radio frequency connected to said control grid and cathode, an anode circuit connecting said anode and cathode, a resistor and condenser connected in series in a common portion of both said grid and anode circuits and an inductance coil connected in shunt across said resistor and condenser said condenser having such a value as to compensate for the losses in said resonant circuit.

4. In an amplifier, the combination of a vacuum tube having a cathode, control grid, anode and screen grid located between said control grid and anode, a tuned grid circuit connecting said control grid and cathode, an anode circuit connecting said anode and cathode, the series connection of a resistor and condenser common to said anode and grid circuits and a connection conductive to direct current between said screen grid and the end of said resistor which is remote from said cathode.

5. In an amplifier, the combination of a vacuum tube having a cathode, an anode and a control grid, a grid circuit resonant to a radio frequency current to be amplified connecting said control grid and cathode, an anode circuit con necting said anode and cathode, a resistor and condenser connected in series to form a common portion of said grid and anode circuits and a coil shunted across said resistor and condenser, said coil having a low impedance to the direct component of the current in said anode circuit and said coil and condenser together being nonresonantat the resonant frequency of said grid circuit.

6. The arrangement of claim 5 in which a switch has its contacts connected to the ends of said coil, said switch being closed to short circuit said coil when said grid circuit is tuned to a long wave-length and in open position when said grid circuit is tuned to a short wave-length.

7. In an arrangement for amplifying high and low radio frequency signal currents, the combination of a vacuum tube having a cathode, a control grid and an anode, a resonant circuit comprising a coil shunted by a condenser connected between said grid and cathode, an anode circuit including an inductance coil connected between said anode and cathode, a resistor and condenser connected in series and arranged to form a path common to said grid and anode circuits, and a pair of switches having a common terminal connected to the low potential end of said resistor and their other terminals connected to respective intermediate points of said coils in the grid and anode circuits, said switches being closed when amplifying currents of high frequency and open when amplifying currents of low frequency.

' KLAAS POS'I'HUMUS. 

